1. Field of the Invention
This invention relates to a laminated structure of compound semiconductors that comprises a IV semiconductor underlying substrate and III-V compound semiconductor layers on the IV semiconductor underlying substrate. More particularly, it relates to a laminated structure of compound semiconductors that attains a reduction of crystal defects in an InP epilayer formed over a Si underlying substrate.
2. Description of the Prior Art
In recent years, thin crystal film growth methods have been remarkably developed by which laser diodes, solar batteries, and high speed devices using two dimensional electron gases, or the like have been produced. However, since these devices are produced by the use of a III-V compound semiconductor as a substrate, their production cost becomes high and the substrate becomes brittle. Moreover, because of difficulties in the growth of crystals, it is difficult to obtain a large-area substrate.
To solve these problems, the heteroepitaxial growth of III-V compound semiconductor on a IV semiconductor underlying substrate has been studied, especially a GaAs thin crystal film is grown on a Si underlying substrate, resulting in a large-area substrate with an excellent crystal quality and low cost. For these conventional crystal film growth methods of GaAs on Si, there are the following three methods, one of which is a two-step growth method in which a GaAs thin film is grown on a Si underlying substrate, first, at a low temperature and then a GaAs film is further grown on the said GaAs thin film at a high temperature (Japanese Laid-Open Patent Publication No. 61-70715), another of which is a method in which a Ge intermediate layer is used between the Si underlying substrate and the GaAs film (IEEE Electron Device Lett. EDL-2, 169 (1981)), and the other is a method in which alternate layers composed of a III-V compound semiconductor (e.g., GaAs) and another III-V group compound semiconductor with a lattice constant that is similar to that of GaAs are used as an intermediate layer disposed between the Si underlying substrate and the GaAs film. By these conventional thin film growth method, field effect transistors, light-emitting diodes, semiconductor laser devices, etc., have been fabricated on an experimental basis. Recently, an excellent crystal quality GaAs thin film has been formed by the use of a strained layer superlattice of InGaAs/GaAs, by which excellent device characteristics are obtainable (Appl. Phys. Lett., 48 (1986 ) 1223).
Since InP, one of the III-V compound semiconductors that have an electron saturation velocity higher than that of GaAs and that have a thermal conductivity greater than that of GaAs, is used instead of GaAs, a microwave power amplifier device that operates at a higher frequency than that with use of GaAs and that produces high output power will be obtainable.
On the other hand, a bulk InP substrate is more expensive than a bulk GaAs substrate, and moreover the InP substrate cannot attain a large crystal area (an area with a diameter of two inches is the largest that has been obtained). Also, crystal quality of InP that are available on the market have a crystal defect density of as great as about 10.sup.4 cm.sup.-2. To overcome these problems, a crystal layer growth method by which an InP layer is grown on a Si underlying substrate has been studied. However, the crystal quality of the InP layer is not yet sufficient and the InP/Si structure is not yet applicable to a device as a substrate, because a difference in the lattice constant between Si and InP is 8.1% and that is about two times that of a difference in the lattice constant between Si and GaAs and because the dissociation pressure of InP is so high that P is liable to be released during the growth of InP and the surface morphology of the InP layer deteriorates, which causes difficulties in an improvement of the crystal quality of the InP layer.
None of the conventional crystal growth methods can reduce not only the large lattice-mismatch between the underlying substrate and the crystal epilayer, but also the stress of the crystal epilayer, and therefore, a high quality crystal layer cannot be formed with reproducibility.